1. Field of the Invention
The invention relates to a method for optical measurement of the three dimensional geometry of objects and is concerned with the task of 3D scanning when there is relative movement between the camera and the object when being scanned, particularly for dental purposes.
2. Prior Art
Digital design data for the computer-assisted production of dental prosthetic items without having to create the usual impression can be acquired by optically scanning one or more teeth in a patient's mouth.
For such scanning, use is frequently made of measuring methods based on the principle of triangulation. Phase-shifting triangulation is disclosed in EP 0 160 797, in which a three-dimensional data set is acquired from a sequence of images of a pattern which is shifted in position from image to image.
With prior triangulation methods, a single straight stripe of light is projected by a projector onto the object being scanned and the projected image is captured by a camera at a parallax angle in the monitoring direction so as to produce an image.
Depending on the surface structure of the object, the light stripe appears no longer to be straight, but curved and offset with respect to its straight form. The surface structure of the object being scanned may then be concluded from the position and shape of the light stripe.
By moving the light stripe transversely to the direction of orientation of the light stripe, the entire object can be scanned to produce images. The altitude data of the scanned object acquired from each individual image can be stored in the form of a two-dimensional pattern in the memory of a computer, which corresponds to a projection of the altitude information of the surface of the scanned object onto a base surface as the reference surface. Following the scanning operation there is obtained a digital, three-dimensional data model of the scanned object, which can be displayed, for example, on a screen as a video still.
A method for the production of a three-dimensional image of an object being scanned is disclosed in WO 2004/085956 A2, a procedure which improves the above-described scanning method in that the surface structure of the scanned object is measured completely during an exposure Pi.
The light stripe can be produced, for example, by the rapid movement of a laser beam throughout the period of image creation such that the light stripe is actually composed of points of light. During an exposure Pi, the pinpoint light beam of a laser is directed over the object being scanned, along a 2-dimensional pattern for the light stripes, in which at least some of the light stripes are substantially parallel to each other, i.e. extend in a direction R1. Space is provided between the light stripes.
During an exposure, the altitude data of the object being scanned are collected along a plurality of parallel, spaced light stripes or sections thereof.
Thus the image obtained already provides the altitude data of the object being scanned in a first group of a plurality of spaced lines of the two-dimensional pattern of the yet to be produced three-dimensional data set.
The independent altitude data acquired from the individual images are merged in the two above-described procedures involving stripe projection which is quasi-static during each exposure, involving one stripe per exposure or a plurality of discrete stripes per exposure, as is known from WO 2004/085956 A2, thus forming an overall image describing or containing all of the 3D data of the object being scanned.
The disadvantage of the described measuring methods involving triangulation is that, due to the fact that a sequence of individual images is created and then a set of at least two images is combined to form an overall image, any camera-shake of the recording apparatus relative to the object between the individual can result in image defects. The position of the recording apparatus relative to the object thus has to remain unchanged when creating a set of images, and can be changed only in the next step for creating a subsequent set of images. An on-the-fly measurement of the object, in which several complete images of the object are created in short periods of time, is thus impossible. The present problem is to provide a method for optical measurement of objects by means of triangulation, particularly for dental purposes, which method is to be capable of allowing on-the-fly measurement.